CSHARP-4756: MongoDB.Bson.BsonSerializationException (when serializing List of MongoDBRef) (#1167)

Author: sanych-sun

src/MongoDB.Bson/IO/BsonWriter.cs

@@ -203,19 +203,13 @@ public void PushSettings(Action<BsonWriterSettings> configurator)
         /// Writes the end of a BSON array to the writer.
         /// </summary>
         public virtual void WriteEndArray()
-        {
-            _serializationDepth--;
-        }
+            => ExitSerializationScope();
 
         /// <summary>
         /// Writes the end of a BSON document to the writer.
         /// </summary>
         public virtual void WriteEndDocument()
-        {
-            _serializationDepth--;
-
-            PopElementNameValidator();
-        }
+            => ExitSerializationScope();
 
         /// <summary>
         /// Writes a BSON Int32 to the writer.
@@ -362,28 +356,13 @@ public virtual void WriteRawBsonDocument(IByteBuffer slice)
         /// Writes the start of a BSON array to the writer.
         /// </summary>
         public virtual void WriteStartArray()
-        {
-            _serializationDepth++;
-            if (_serializationDepth > _settings.MaxSerializationDepth)
-            {
-                throw new BsonSerializationException("Maximum serialization depth exceeded (does the object being serialized have a circular reference?).");
-            }
-        }
+            => EnterSerializationScope();
 
         /// <summary>
         /// Writes the start of a BSON document to the writer.
         /// </summary>
         public virtual void WriteStartDocument()
-        {
-            _serializationDepth++;
-            if (_serializationDepth > _settings.MaxSerializationDepth)
-            {
-                throw new BsonSerializationException("Maximum serialization depth exceeded (does the object being serialized have a circular reference?).");
-            }
-
-            var childValidator = _useChildValidator ? _elementNameValidator.GetValidatorForChildContent(_name) : _elementNameValidator;
-            PushElementNameValidator(childValidator);
-        }
+            => EnterSerializationScope();
 
         /// <summary>
         /// Writes a BSON String to the writer.
@@ -470,5 +449,23 @@ protected void ThrowInvalidState(string methodName, params BsonWriterState[] val
                 methodName, validStatesString, _state);
             throw new InvalidOperationException(message);
         }
+
+        private void EnterSerializationScope()
+        {
+            _serializationDepth++;
+            if (_serializationDepth > _settings.MaxSerializationDepth)
+            {
+                throw new BsonSerializationException("Maximum serialization depth exceeded (does the object being serialized have a circular reference?).");
+            }
+
+            var childValidator = _useChildValidator ? _elementNameValidator.GetValidatorForChildContent(_name) : _elementNameValidator;
+            PushElementNameValidator(childValidator);
+        }
+
+        private void ExitSerializationScope()
+        {
+            _serializationDepth--;
+            PopElementNameValidator();
+        }
     }
 }

tests/MongoDB.Bson.Tests/IO/BsonWriterTests.cs

@@ -166,6 +166,46 @@ public void BsonWriter_should_validate_child_elements_names(
             }
         }
 
+        [Theory]
+        [InlineData(0)]
+        [InlineData(1)]
+        [InlineData(2)]
+        [InlineData(10)]
+        public void BsonWriter_array_should_use_own_element_validator(int arraySize)
+        {
+            var arrayItemValidator = CreatePrefixValidatorMock("$");
+            var rootValidator = CreatePrefixValidatorMock("#", arrayItemValidator.Object);
+
+            using (var stream = new MemoryStream())
+            using (var writer = new BsonBinaryWriter(stream))
+            {
+                writer.WriteStartDocument();
+                writer.PushElementNameValidator(rootValidator.Object);
+
+                writer.WriteName("#array");
+                writer.WriteStartArray();
+
+                for (var i = 0; i < arraySize; i++)
+                {
+                    writer.WriteStartDocument();
+
+                    writer.WriteName("$a");
+                    writer.WriteInt32(i);
+
+                    writer.WriteEndDocument();
+                }
+
+                writer.WriteEndArray();
+
+                writer.WriteName("#value");
+                writer.WriteInt32(arraySize);
+                writer.WriteEndDocument();
+            }
+
+            rootValidator.Verify(v => v.IsValidElementName(It.IsAny<string>()), Times.Exactly(2));
+            arrayItemValidator.Verify(v => v.IsValidElementName(It.IsAny<string>()), Times.Exactly(arraySize));
+        }
+
         // private methods
         private Mock<IElementNameValidator>[] CreatePrefixValidatorsNestedMock(string[] prefixes)
         {